The U.S. Pat. No. 3,774,193 to Restrepo, teaches that by means of a portable unit which contains a double selector, one to select an internal impedance value which is to be compared, by means of an external plug of the portable unit, with an impedance value contained in a corresponding fixed station; these values, when determined to be equal by means of a comparison using a Wheastone Bridge cause an associated electronic circuit to permit the selector advance to the following position so that a new comparison with the following station and thus successively up to the last station, can take place with which everything is disposed to send a reset singal which upon reception by one of the circuits of the master fixed unit determines that the time accumulated in an adjustable timer of the fixed unit returns to zero, avoiding thus completion of the programmed time in the timer of the master unit, and avoiding thus an alarm which can be activated by the master unit and that the fault be registered in a counter, with the previous activation of a prealarm signal and the succeeding retarded activation of a general alarm.
A second selector in the portable unit is synchronized with the first selector and permits establishing the minimum time which the watchman must employ between one station and the following.
The described system requires a portable unit and a master unit in a fixed place and the alarms due to the nonfulfilment of the watchman in his tour can only be produced in fixed places, frequently not audible by the watchman. Moreover, the control stations require a connector with which good contact may fail as a result of the bad weather, and the water and moisture may vary the external impedance used in comparison, facilitating an erratic functioning. The heavy duty to which the portable units normally are subjected produces frequently the deterioration of the plug and associated cable.
There exist other systems like that disclosed in the U.S. Pat. No. 3,579,221 to Ashley in which a supervisor can monitor the tour followed by the watchman by observing lights in a panel. Such a system is substantially different to the proposed one because, being wireless, requires sending modulated signals of radio frequency.
Other systems such as that of the U.S. Pat. No. 4,024,527 to Houghton, even if they accomplish functions similar to the proposed one, require wiring to provide interconnecton between the control stations and the master control.